The present invention relates to a polyvinyl chloride (PVC) foam material, and more particularly to a rigid PVC foam composite material, which is made without using an aiding agent such as the fluoro chloro carbon.
The polyvinyl chloride is in itself not a suitable substance for use as a foam material, in view of the facts that the molecular bridging of PVC is difficult to establish and that PVC has a relatively low viscosity even in a molten state. Therefore, the prior art method of making PVC foam material makes use of the fluoro chloro carbon as an agent to help increase the viscosity of PVC, as exemplified in an European Patent Application 0039117A1. Such prior art method includes an addition of 0.1-0.5 part by weight of alkyl methacrylate to 100 parts by weight of PVC, and an addition of the polymerized methyl methacrylate (PMMA) to the alkyl group with 1-10 carbon atoms. Such mixture is then fed into a double-screw extruder by which the mixture is granulated. The granules so made are mixed with an excess of trichloro fluoro methane and are stirred for 22 hours to ensure that the granules are impregnated with trichloro fluoro methane. The impregnated granules are subsequently mixed with one part by weight of citric acid powder per 100 parts by weight of PVC. This mixture so obtained is fed into a single-screw extruder in which the mixture is heated and pressurized so as to foam.
The chemical substance of trichloro methane used as a physical foaming agent in the prior art method described above is a potential source of environmental pollution and is therefore likely to be banned. In addition, the cost of a single-screw extruder and a double-screw extruder is rather high. Furthermore, the prior art method includes a process in which the granules are stirred in trichloro fluoro methane for 22 hours. As a result, the prior art method is not economically feasible.